How big a percentage of the above-mentioned user group does not speak
English?

Note that it's about being able to read/speak/write English, it has
nothing to do with "US" or not.

My personal belief is that most of the internationalization efforts are of
very low priority for that particular user group.

And you are very wrong.
A lot of people can live with english. That doesn't mean they would not
jump on a localized version if available. Didn't you notice the efforts
an ubber-geek like Alan Cox spent on Welsh lately ? Yet he can live with
english too.

Non native langage apps are a pain just like non AA fonts were or XFree
approximative drivers for new gfx cards are. Do not underestimate the
impact of little pains - a project like Gnome saw the HiG light after
accumulating for years "low priority" problems people "could live with".